In less than five weeks, New Horizons will zip past the Pluto-Charon system in a brief but historic encounter. Given the huge interest in Pluto, it’s fair to ask: Why won’t mission planners let the probe hang out a while?

Above: Our first glimpse of Pluto and Charon in color (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The simple reason is that New Horizons can’t make a stop at the Pluto-Charon system. It’s a constraint that has as much to do with engineering as it does with basic physics.

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Above and below: Pluto’s position as of today, June 9, 2015. (NASA/New Horizons)

In order to get New Horizons to Pluto in a reasonable amount of time (in this case 9.5 years), NASA had to get the probe moving very, very fast. And a probe on the move can be difficult to slow down.

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After its launch from Cape Canaveral on January 19, 2006, the probe entered into an escape trajectory featuring a speed of 16.26 kilometers per second (58,536 km/h; 36,373 mph), setting a new record for the highest launch speed of a human-made object flung from Earth. New Horizons’s encounter with Jupiter offered a subsequent gravitational assist that increased its speed by an additional 4 km/s (14,000 km/h; 9,000 mph). Once at the Pluto-Charon system, the spacecraft will pass through at a velocity of about 13.8 km/s relative to the dwarf planet (49,680 km/h; 30,800 mph).

That’s obviously a lot of momentum. To get New Horizons into Pluto’s orbit, mission planners would have to reduce its speed by over 90%, which would require more than 1,000 times the amount of fuel the probe can carry. That’s a technologically unfeasible proposition. And so, the probe will have no choice but to zoom past Pluto, feverishly snapping pics and taking measurements before being flung outward towards the Kuiper belt.

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Which is a pretty neat consolation prize. The New Horizons mission will be far from over after its July 14 encounter with Pluto.